1. Field of the Invention
The present invention relates to a method for producing a carbonic ester from an organometal compound and carbon dioxide. More particularly, the present invention is concerned with a method for producing a carbonic ester, comprising (1) performing a reaction between an organometal compound having a metal-oxygen-carbon linkage and carbon dioxide to obtain a reaction mixture containing a carbonic ester formed by the reaction, (2) separating the carbonic ester from the reaction mixture to obtain a residual liquid, and (3) reacting the residual liquid with an alcohol to form an organometal compound having a metal-oxygen-carbon linkage and form water and removing the water from the organometal compound, wherein the organometal compound obtained in step (3) is recovered for recycle thereof to step (1). By the method of the present invention, a carbonic ester can be produced in high yield from an organometal compound having a metal-oxygen-carbon linkage and carbon dioxide. It is advantageous that carbon dioxide has no toxicity and no corrosiveness and is inexpensive. Further, the method of the present invention is advantageous not only in that the organometal compound after use in this method can be regenerated and recycled to step (1) of the method, thereby preventing occurrence of wastes derived from the organometal compound, but also in that there is no need for the use of a large amount of a dehydrating agent, thereby preventing occurrence of wastes derived from the dehydrating agent. Therefore, the method of the present invention is commercially very useful and has high commercial value.
2. Prior Art
A carbonic ester is a useful compound. For example, a carbonic ester is used as additives for various purposes, such as a gasoline additive for improving the octane number of a gasoline, and a diesel fuel additive for reducing the amount of particles in an exhaust gas generated by the burning of a diesel fuel. A carbonic ester is also used as an alkylation agent, a carbonylation agent, a solvent and the like in the field of the synthesis of organic compounds, such as polycarbonate, urethane, pharmaceuticals and agrichemicals. A carbonic ester is also used as an electrolyte for a lithium battery, a raw material for producing a lubricant oil and a raw material for producing a deoxidizer which can be used for preventing boiler pipes from rusting.
As a conventional method for producing a carbonic ester, there can be mentioned a method in which phosgene as a carbonyl source is reacted with an alcohol, thereby producing a carbonic ester. Since phosgene used in this method is extremely harmful and highly corrosive, this method is disadvantageous in that the transportation and storage of phosgene need minute care and, also, a large cost is needed for the maintenance of production equipment and for assuring safety. Further, this method poses a problem in that it is necessary to dispose of hydrochloric acid, which is by-produced, as a waste.
As another conventional method for producing a carbonic ester, there is known an oxidative carbonylation method in which carbon monoxide as a carbonyl source is reacted with an alcohol and oxygen in the presence of a catalyst, such as copper chloride, thereby producing a carbonic ester. In this method, carbon monoxide (which is extremely harmful) is used under high pressure; therefore, this method is disadvantageous in that a large cost is needed for the maintenance of production equipment and for assuring safety. In addition, this method poses a problem in that a side reaction occurs, such as oxidation of carbon monoxide to form carbon dioxide. For these reasons, it has been desired to develop a safer method for producing a carbonic ester.
In these conventional methods in which phosgene or carbon monoxide is used as a raw material, a halogen, such as chlorine, is contained in the raw material itself or in the catalyst used. Therefore, in the case of these methods, a carbonic ester obtained contains a trace amount of a halogen which cannot be completely removed by a purification step. When such carbonic ester is used as a gasoline additive, a light oil additive or a material for producing electronic equipment, it is possible that the halogen contained in the carbonic ester causes corrosion of equipment. For reducing the amount of a halogen in the carbonic ester to an extremely trace amount, it is necessary to perform a thorough purification of the carbonic ester. For this reason, it has been desired to develop a method for producing a carbonic ester, which does not use any of a halogen-containing raw material and a halogen-containing catalyst.
On the other hand, a method has been put to practical use, in which carbon dioxide is reacted with ethylene oxide or the like to obtain a cyclic carbonic ester, and the obtained cyclic carbonic ester is reacted with methanol, thereby producing dimethyl carbonate. This method is advantageous in that carbon dioxide as a raw material is harmless, and a corrosive substance, such as hydrochloric acid, is substantially not used or generated. However, this method poses the following problems. Ethylene glycol is by-produced in this method; therefore, from the viewpoint of cost reduction, it is necessary to find ways to effectively utilize the by-produced ethylene glycol. Further, it is difficult to perform safe transportation of ethylene (which is a raw material for producing ethylene oxide) and ethylene oxide. Therefore, for removing the need for the transportation, it is necessary that a plant for producing a carbonic ester by this method be built at a location which is adjacent to a plant for producing ethylene and ethylene oxide.
There is also known a method in which carbon dioxide as a carbonyl source is subjected to an equilibrium reaction with an alcohol in the presence of a catalyst comprising an organometal compound having a metal-oxygen-carbon linkage, thereby forming a carbonic ester and water. This equilibrium reaction is represented by the following formula (3):
(R Represents an Unsaturated or Saturated Hydrocarbon Group)This method is advantageous in that carbon dioxide and an alcohol as raw materials are harmless. However, this method employs an equilibrium reaction in which a carbonic ester and water are simultaneously formed as products. Also in the case of the above-mentioned oxidative carbonylation method using carbon monoxide, water is formed. However, the formation of water in the equilibrium reaction is totally different in significance from the formation of water in the oxidative carbonylation, which is not an equilibrium reaction. The equilibrium of an equilibrium reaction using carbon dioxide as a raw material is thermodynamically biased toward the original system. Therefore, the method using an equilibrium reaction has a problem in that, for producing a carbonic ester in high yield, it is necessary that a carbonic ester and water as products be removed from the reaction system. Further, there is also a problem in that the water formed decomposes a catalyst, so that not only is the reaction hindered, but also the number of turnovers of a catalyst (i.e., number of turnovers of a catalyst (i.e., the number of cycles of regeneration and reuse) is only 2 or 3. For solving this problem, various methods for removing water (which is a product) by using a dehydrating agent have been proposed.
For example, there has been proposed a method in which an alcohol and carbon dioxide are reacted with each other in the presence of a metal alkoxide as a catalyst, thereby forming a carbonic ester and water, wherein a large amount of dicyclohexylcarbodiimide (DCC) (which is an expensive organic dehydrating agent) or the like is used as a dehydrating agent (see Collect. Czech. Chem. Commun. Vol. 60, 687-692 (1995)). This method has a problem in that the dehydrating agent after use cannot be regenerated, resulting in the occurrence of a large amount of a waste derived from the dehydrating agent.
There is known a method for producing a carbonic ester, in which a carboxylic acid orthoester is used as an organic dehydrating agent (see Unexamined Japanese Patent Application Laid-Open Specification No. Hei 11-35521). (In this patent document, there are descriptions reading: “a carboxylic acid orthoester is reacted with carbon dioxide” and “an acetal is reacted with carbon dioxide”. However, as a result of recent studies in the art, it is generally presumed that the actual reaction route is as follows. “An alcohol and carbon dioxide are reacted with each other to obtain a carbonic ester and water. A part of the water is reacted with a carboxylic acid orthoester. The remainder of the water is reacted with acetal.”) This method has problems in that a carboxylic acid orthoester (which is an expensive compound) is used as a dehydrating agent, and methyl acetate is known to be by-produced (see “Kagaku Sochi (Chemical Equipment)”, Vol. 41, No. 2, 52-54 (1999)). Thus, this method is as defective as the above-mentioned method.
Further, there has been proposed a method in which a large amount of an acetal compound is used as an organic dehydrating agent (see German Patent No. 4310109). There is also a patent document in which it is described that an acetal and carbon dioxide are reacted with each other by using, as a catalyst, a metal alkoxide or dibutyltin oxide (see Unexamined Japanese Patent Application Laid-Open Specification No. 2001-31629). (With respect to the reaction described in the latter, as a result of recent studies in the art, it is generally presumed that the actual reaction route is as follows. “An alcohol and carbon dioxide are reacted with each other to obtain a carbonic ester and water. The water is then reacted with an acetal.”) However, these patent documents do not teach or suggest a method for producing an acetal compound in high yield without forming a waste. Further, the methods disclosed in these patent documents have a problem in that, when an acetal compound is used as a dehydrating agent, large amounts of by-products, such as a ketone and an aldehyde, are formed as wastes.
The effects aimed at by the methods which employ an organic dehydrating agent are to improve the number of turnovers of a catalyst. However, an organic dehydrating agent is consumed in a stoichiometric amount in accordance with the formation of a carbonic ester (and water as a by-product), so that a large amount of an organic dehydrating agent is consumed, thus forming a large amount of a degeneration product of the organic dehydrating agent. Therefore, it is necessary to perform an additional step of regenerating a large amount of a degenerated organic dehydrating agent. Further, in spite of the use of an organic dehydrating agent, the possibility still remains that deactivation of a catalyst occurs. The reason is as follows. In the conventional method for producing a carbonic ester by using the equilibrium reaction of the above-mentioned formula (3), carbon dioxide is in a supercritical state and, hence, the reaction is performed in supercritical carbon dioxide. In general, in supercritical carbon dioxide, a catalyst exhibits poor solubility, and the catalyst particles are likely to cohere together.
Therefore, there is a problem in that, when an organotin compound (which is susceptive to polymerization) is used as a catalyst in supercritical carbon dioxide, the organotin compound as a catalyst is likely to be deactivated due to its polymerization.
There has also been proposed a method which employs a solid dehydrating agent (see Applied Catalysts Vol. 142, L1-L3 (1996)). However, this method has a problem in that the solid dehydrating agent cannot be regenerated, thus forming a large amount of a waste.
There is also known a method in which an alcohol (methanol) and carbon dioxide are reacted with each other in the presence of a metal oxide (dibutyltin oxide) to thereby obtain a reaction mixture, and the obtained reaction mixture is cooled and introduced into a packed column containing a solid dehydrating agent, thereby gradually displacing the equilibrium toward a carbonic ester while effecting dehydration, to obtain a carbonic ester (see Unexamined Japanese Patent Application Laid-Open Specification No. 2001-247519). This method is based on a technique in which a conventional technique of using a dehydrating agent is combined with the known phenomenon that the water adsorbability of a conventional dehydrating agent (such as a molecular sieve) exhibits a temperature dependency. A dehydrating agent (such as a molecular sieve) exhibits lower water adsorbability at high temperatures than at low temperatures. Therefore, for removing a trace amount of water (by-product) from a reaction mixture which contains a largely excess amount of a low molecular weight alcohol used as a solvent, it is necessary to cool the reaction mixture in which an equilibrium is achieved under high temperature and pressure conditions, before introducing the reaction mixture into a packed column containing a solid dehydrating agent. In addition, for increasing the conversion of an alcohol as a raw material, it is necessary that the reaction mixture which has been cooled and dehydrated in the packed column be returned to high temperature and pressure conditions which are necessary for the reaction. Thus, this method has problems in that it is necessary to consume an extremely large amount of energy for cooling and heating, and a large amount of a solid dehydrating agent is needed. This method is very widely used for producing an aliphatic ester having a relatively large equilibrium constant. However, in the production of a carbonic ester from carbon dioxide and an alcohol, wherein the equilibrium of the reaction is largely biased toward the original system, this method cannot be suitably used because this method poses a serious problem that it is necessary to repeat the above-mentioned operation which needs a very large consumption of energy for cooling and heating. Further, for regenerating a degenerated dehydrating agent which has adsorbed water to saturation, it is generally necessary to calcine the degenerated dehydrating agent at several hundreds° C., thus rendering this method commercially disadvantageous. Furthermore, in this method, only one (water) of the two products of an equilibrium reaction is removed and, therefore, there is a problem in that, when the equilibrium reaction progresses to increase the carbonic ester concentration of the reaction system, the reaction becomes unlikely to progress any more, that is, this method is still under the restriction of an equilibrium reaction. In addition, dibutyltin oxide, which is used as a catalyst in this method, exhibits an extremely poor solubility in methanol and, hence, almost all of dibutyltin oxide as a catalyst remains in solid form in the reaction mixture. Therefore, when the reaction mixture is cooled to room temperature in a cooling step, the reaction mixture turns into a white slurry, thus causing a problem in that, in a subsequent dehydration step performed using a packed column containing a dehydrating agent, the slurry causes clogging of the packed column.
In general, a dehydration method in which water is removed by distillation is well-known in the field of organic synthesis reactions. However, in the field of the production of a carbonic ester from carbon dioxide and an alcohol, although “Study Report of Asahi Glass Association for Promotion of Industrial Technology (Asahi Garasu Kogyogijutsu Shoreikai Kenkyu Hokoku)”, Vol. 33, 31-45 (1978) states that “dehydration by distillation is now being studied”, there have been no reports or the like which state that a dehydration method using distillation has been completed. The reason why dehydration by distillation has not been performed in the art is, for example, that it is known that, when distillation under heating is performed, a reverse reaction occurs, causing a loss of a carbonic ester (see “Journal of the Chemical Society of Japan (Nippon Kagaku Kaishi)”, No. 10, 1789-1794 (1975)). It is conceivable that, for lowering the heating temperature for distillation, the distillation is performed under reduced pressure. However, in the field of the distillation engineering, it is a common knowledge that it is difficult to completely remove a trace amount of water, by simple distillation, from a solvent having a hydrophilic group, such as an alcohol. Therefore, as dehydration methods, no dehydration method is known other than those methods using a large amount of an organic dehydrating agent or a large amount of a solid dehydrating agent. That is, all known dehydration methods have problems in that a large amount of a waste occurs, or a large consumption of energy is inevitable.
There has been a report which mentions a distillation separation of a carbonic ester from a reaction mixture containing a metal alkoxide, wherein the reaction mixture is obtained by reacting carbon dioxide and an alcohol with each other in the presence of a metal alkoxide catalyst; however, it is known in the art that, when a metal alkoxide catalyst is used, a distillation separation causes a reverse reaction, thus rendering it difficult to recover a carbonic ester by distillation separation (see “Journal of the Chemical Society of Japan (Nippon Kagaku Kaishi)”, No. 10, 1789-1794 (1975)). Especially, no method is known by which a carbonic ester having a high boiling point can be separated in high yield from a reaction mixture containing a metal alkoxide.
A metal alkoxide is so unstable that it is susceptive to deactivation due to the moisture in the air. Therefore, the handling of a metal alkoxide needs strict care. For this reason, no conventional technique using a metal alkoxide catalyst has been employed in the commercial production of a carbonic ester. A metal alkoxide catalyst is an expensive compound, and no technique is known for regenerating a deactivated metal alkoxide catalyst.
There has been proposed a method for producing a carbonic ester by using a dibutyltin dialkoxide as a catalyst, in which, during the reaction, the catalyst is formed from dibutyltin oxide (which is stable to moisture) added to the reaction system (see Japanese Patent No. 3128576). This method has a problem in that, although dibutyltin oxide which is charged into the reaction system is stable, the dibutyltin oxide is converted, during the reaction, into a dibutyltin dialkoxide, which is unstable. Therefore, this method cannot solve the above-mentioned problem of the instability of a metal alkoxide catalyst. Further, for converting dibutyltin oxide into a dibutyltin dialkoxide in a reaction system, the reaction system needs to be placed under high temperature and pressure conditions. The reason for this is as follows. When an alkoxide is formed from dibutyltin oxide, water is generated, and the water needs to be consumed by hydrolysis of an acetal. However, tin exhibits only a very weak acidity, so that high temperature and pressure conditions are necessary for catalyzing the above-mentioned hydrolysis of an acetal.
Thus, in the conventional methods for producing a carbonic ester by using a metal alkoxide, carbon dioxide and an alcohol, when the metal alkoxide (which is expensive) has lost its catalyst activity due to hydrolysis or the like, there is no way to easily and effectively regenerate and reuse the metal alkoxide. Therefore, the conventional methods for producing a carbonic ester is disadvantageous in that it is necessary to use a large amount of an organic dehydrating agent or a solid dehydrating agent in combination with a small amount of a metal alkoxide.
As described hereinabove, the prior art techniques for producing a carbonic ester have many problems and, therefore, have not been put to practical use.